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For a planar point set we consider the graph whose vertices are the crossing-free straightline spanning trees of the point set, and two such spanning trees are adjacent if their union is crossing-free. An upper bound on the diameter of this graph implies an upper bound on the diameter of the flip graph of pseudo-triangulations of the underlying point set.We prove a lower bound of Ω(log n/ log log n) for the diameter of the transformation graph of spanning trees on a set of n points in the plane. This nearly matches the known upper bound of O (log n). If we measure the diameter in terms of the number of convex layers k of the point set, our lower bound construction is tight, i.e., the diameter is in Ω(log k) which matches the known upper bound of O (log k). So far only constant lower bounds were known.

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